BPS2026 – Reconstituting the liquid-liquid interface of chromatin condensates

Heterochromatin is an essential component of eukaryotic genomes responsible for genome integrity and regulation and is defined by enrichment for Histone3 lysine 9-methylation (H3K9me3), and its recognition by the reader protein heterochromatin protein 1 (HP1a). Loss of either HP1a or H3K9me3 leads to a loss of transcriptional silencing, mitotic failure, loss of nuclear compartmentalization, and large-scale defects in nuclear morphology, indicating that heterochromatin plays a pleotropic role in regulating and organizing the genome. Using a minimal system of well-ordered chromatin arrays, with and without H3K9me3 and HP1a, we have built an in vitro system that results in the spontaneous formation of discrete and tunable interfaces separating unmodified chromatin from methylated chromatin. To probe the nature of these interfaces, we characterized the effects of four distinct HP1a mutants and found that a delicate balance of interactions between HP1a and the underlying chromatin determines the relative interfacial tensions between chromatin compartments. By analyzing the geometry and fidelity of these liquid-liquid interfaces, we have developed a working model that chromatin solvation is modulated by both H3K9me3 and HP1a and plays a critical role determining biochemical transactions along chromatin.